a) Field of the Invention
The invention is directed to an arrangement and to the associated process for cleaning exhaust gases with oxidizable pollutants in which even small concentrations of pollutants can be safely reduced through the combination of adsorption and catalysis with low expenditure of energy. The invention is further directed to the development of the process as a regenerative principle.
b) Description of the Related Art
A large number of modern processes for the treatment and processing of polymer materials and the melting losses or burnup of oils and other process materials in metalworking generate polluted exhaust air that is usually removed from the workplace by means of plant exhaust installations or localized exhaust systems in order to maintain work hygiene standards. Similar exhaust installations are used in kitchen facilities to remove steam or vapor especially when foodstuff is being prepared by frying in oils and fats.
All known processes have in common that a considerable amount of heat is extracted from the room air by the exhaust means. For this reason, exhaust installations of this type often contain costly air-to-air heat exchangers or else the heat is supplied subsequently by additional heating of the air supply. In the prior art, filter units and separating units which are combined in various ways in accordance with materials processing techniques ensure that necessary emission standards with respect to the introduction of exhaust gases into the atmosphere are maintained. For the most part, the residues of the filter units and separating units must be disposed of as special waste in a costly manner.
In the case of exhaust gases containing toxic pollutants, it is generally not possible to reintroduce the filtered exhaust air back into the workplace, since there are hardly any economically feasible solutions for safely removing pollutants with an adequate cleaning effect.
Cleaning requirements are especially difficult when the separated residue has multiple phases and its solid phase has a sticky, pasty consistency. For these reasons, wet scrubbers are frequently used for laser material processing plants (see DE 3 203 908 A1 and DE 4 422 042 A1).
The starting point for the solution to this problem consists in the use of catalytic exhaust gas treatment reactors. The vast majority of known technical solutions for exhaust gas catalysis have been for the treatment of exhaust gases of Otto engines and diesel engines. Besides the known precious-metal-containing catalysts on ceramic carriers, catalysts are shown, e.g., in DE 3 940 758 C2, whose active components include vanadium and platinum metals which are applied to finely particulate oxides or zeolites. A further development of this principle is shown in DE 4 213 018, wherein the actual catalyst carrier is applied to a monolithic catalyst body with through-flow channels by coating, this catalyst carrier being provided in turn with the active components which are already known. Also known from DE 3 325 292 A1 are oxide carrier catalysts which do not contain any precious metals and are based on iron, nickel and cobalt. As an alternative to these oxide carrier catalysts, technical solutions are described for catalytically active systems of wire wool, woven wire or knitted wire (DE 4 243 500 A1, DE 4 417 984 C1). Principles for the production of such catalysts are listed in DE 4 416 469 C1, DE 19 508 820 C1, DE 19 503 865 C1, DE 19 507 179 C1, and DE 19 539 827 C1.
The combination of catalytic reactors with prior adsorption is described in DE 4 142 176 A1 for cleaning exhaust air from ceramic ovens. For this purpose, the exhaust air is guided successively through an adsorber honeycomb, an electrically operated gas heater, and a honeycomb catalyst. A more complex design is pursued in DE 19 527 490 A1 in that the adsorber is arranged subsequent to the catalyst, the reaction product of the catalyst flows through the adsorber in the course of operation, and the adsorber is subjected to a desorption step at intervals. The pollutants which are liberated in so doing are guided back to the catalyst again through a return channel, while the reaction product of the catalyst is released without secondary treatment. The separation of different pollutants in exhaust gases by means of partial desorption of an adsorber with the aim of selectively eliminating these pollutants is shown in EP 9 101 367. This invention is further directed to the associated principle for returning gas.
Arrangements having the aim of protecting the catalyst against deposits and catalyst toxins are also known from the automotive field. DE 4 326 121 C2 shows a technical solution in which the prior adsorption is carried out with zeolites which are resistant to high temperatures. Further, DE 3 407 172 C2 describes a principle for cleaning the exhaust gases of diesel engines in which filter elements for soot alternate with catalyst elements for post-combustion of gaseous components. For this purpose, the soot filter elements are also outfitted with a catalyst which lowers the firing temperature of the soot and promotes its burnup. U.S. Pat. No. 5,272,874 refers to a combination of catalysts, heat exchangers and filters to achieve a good filter efficiency of the filter downstream thereof by means of cooling the reaction products of the catalyst.
The known technical solutions for oxidative exhaust gas cleaning concentrate on the treatment of relatively high pollutant concentrations to achieve an autothermal operating mode of the catalyst. In general, external heating of the exhaust air for the purpose of achieving and maintaining the working temperature of the catalyst is uneconomical. Where the elimination of unacceptable concentrations of pollutants with respect to work hygiene is concerned, a concentration sufficient for the autothermal catalyst operation is never achieved.
A further disadvantage in the known arrangements and processes consists in that a cleaning efficiency allowing the exhaust air to be reintroduced into the workplace is achieved only by the most elaborate combinations of processes. In such cases, washing liquids or solid filter auxiliary materials are needed, which further increases the quantity of special waste generated.
There is a need, especially in consideration of the use of new materials, for processes for cleaning waste air which have a high cleaning efficiency without burdening the heat-energy balance in the workplace. Further, the amount of special waste is minimized. The process endeavors to make do without substantial preheating of the main air flow.